1. Prepare the turkey: Preheat the oven to 400° Set the turkey breast side up in a large roasting pan. Fold the wing tips under the bird and season the cavity with salt and ground pepper. Rub the bird generously with vegetable oil and season with salt and ground pepper. Roast the turkey for 20 minutes.
2. In a bowl, combine 2 cups of the chicken stock with the 2 tablespoons of vegetable oil. Baste the turkey with the stock and vegetable oil mixture and scatter the chopped onion, carrot and celery around the turkey in the pan. Lower the oven temperature to 325° and roast the turkey for 3 1/2 hours longer, basting the turkey skin with the stock mixture every 30 minutes and rotating the pan several times during the cooking, until an instant-read thermometer inserted in the inner thigh of the turkey registers 165°.
3. Meanwhile, rub the turkey neck with vegetable oil and season it with salt and black pepper. Heat a saucepan over moderately high heat. Add the turkey neck and cook until it is starting to brown on the first side, about 3 minutes. Turn the neck, lower the heat to moderate and cook until browned on the second side, 10 minutes. Pour the remaining 6 cups of stock into the saucepan and bring to a boil over high heat. Cover; reduce the heat to low and simmer, skimming frequently, for 2 hours. Remove and discard the neck; you should have about 4 1/2 cups of stock.
4. Make the glaze: In another saucepan, combine 3 cups of the fresh tangerine juice with the grated ginger and sugar. Bring the mixture to a boil over moderately high heat. In a small bowl, blend the cornstarch with the remaining 3 tablespoons of tangerine juice until smooth. Whisk the cornstarch mixture into the tangerine juice mixture in the saucepan and bring the liquid to a boil, whisking constantly, until it has thickened, about 2 minutes. Take the saucepan off the heat, stir in the chopped sage leaves and season with salt; let the glaze cool.
5. When the turkey is done, remove it from the oven. Increase the oven temperature to 400°. Generously spoon half of the tangerine glaze over the turkey and roast for about 10 minutes, or until the skin starts to brown. Coat the turkey skin with the remaining tangerine glaze and bake the turkey for about 10 minutes longer, or until the skin is richly browned. Transfer the turkey to a large carving board and let rest for 15 to 25 minutes.
6. Meanwhile, set the roasting pan over 2 burners on moderately high heat. Add 1 cup of the turkey stock and boil, stirring with a sturdy wooden spoon and scraping up the browned bits. Set a mesh strainer over a medium saucepan. Carefully pour the roasting pan juices into the strainer, pressing down on the vegetables to extract as much of the pan juices as possible. Discard the vegetables.
7. Skim the fat from the pan juices, reserving 1/4 cup of the fat in a small bowl and discarding the rest. Whisk the flour into the fat to make a smooth paste. Gradually whisk the flour paste into the pan juices in the saucepan and bring to a boil, whisking constantly. Simmer until the liquid is thickened, about 2 minutes, then whisk in the remaining 3 1/2 cups of turkey stock. Simmer over moderately low heat, whisking often, until the gravy is thickened, about 10 minutes. Season with salt and pepper and transfer to a gravy boat. Carve the turkey and serve, passing the gravy.